The effects of forskolin and calcium ionophore A23187 on secretion and cytoplasmic RNA levels of Chromogranin-A and calcitonin

Hormone storage vesicle proteins. Transcriptional basis of the widespread neuroendocrine expression of chromogranin A, and evidence of its diverse biological actions, intracellular and extracellular

Chromogranin A (CgA) is an acidic soluble protein found in the core of secretory vesicles throughout the neuroendocrine system, from which it is coreleased by exocytosis with a variety of amine and peptide hormones and neurotransmitters. Much has now been learned about the structure of CgA, and there is emerging evidence that it plays several biological roles, both within secretory granules and after release from neuroendocrine cells. Factors governing its gene's widespread yet restricted (neuroendocrine) pattern of expression are only now being explored. In an attempt to understand how cells throughout the neuroendocrine system (but not exocrine or other nonendocrine cells) turn on and control the expression of CgA, we have isolated and begun to characterize functional 5' promoter elements from the rodent CgA genes. Within the sympathoadrenal system, interest focuses on a recently proposed (though as yet incompletely investigated) function of CgA: its ability to suppress catecholamine release from adrenal chromaffin cells when such cells are stimulated by their usual physiologic secretagogue. We anticipate that such studies will contribute to an understanding of this abundant, yet previously mysterious protein's role in neuroendocrine function.

Cyclic AMP increases the release of parathyroid hormone-related protein from a lung-cancer cell line

Parathyroid hormone-related protein (PTHrP) plays an important role in the pathogenesis of malignant hypercalcemia by stimulating bone resorption and/or renal tubular reabsorption of calcium. In cultured cancer cells, its production can be influenced by various factors or ions, but the regulation of its production is still poorly understood. We investigated the effects of stimulators of cAMP synthesis on PTHrP release by a human lung squamous-carcinoma cell line (BEN). In superfused cells grown on microcarrier beads, PTHrP production was significantly increased after incubation with calcitonin for only 20 min. The release of immunoreactive and bioactive PTHrP was increased by incubating the cells with forskolin, 3-isobutyl-1-methylxanthine or dibutyryl cAMP even in the presence of the protein-synthesis inhibitor cycloheximide for 6 hr. The calcitonin-mediated stimulation was not accompanied by concomitant changes in PTHrP mRNA. The microfilament-disrupter cytochalasin D was shown to enhance the basal and calcitonin-induced production of PTHrP. These results indicate that stimulators of cAMP synthesis enhanced PTHrP release by BEN cells.

Differential regulation of chromogranin B/secretogranin I and secretogranin II by forskolin in PC12 cells

The factors which regulate the expression of the granin family of secretory proteins have yet to be completely described. The present study investigated the effects of forskolin (FSK), an activator of adenylate cyclase, on the regulation of chromogranin B/secretogranin I (CgB) and secretogranin II (SgII) mRNA levels in rat PC12 cells. PC12 cells were treated with 10 microM FSK for time points up to 48 h and were harvested for cAMP determination, RNA isolation and Northern blot analysis, or fixed in 4% paraformaldehyde for immunocytochemistry. Cellular cAMP levels peaked after two h of FSK treatment and remained elevated for 48 h. Chromogranin B mRNA increased with FSK treatment, reaching a maximum of 7-fold above control after 24 h, while the level of SgII mRNA decreased to a level of 65 +/- 10% of control after 48 h. The effects of FSK on CgB mRNA appear to be mediated by cAMP, as 8-bromo-cAMP (500 microM) resulted in a 2.8-fold increase in CgB mRNA, and H-89 (30 microM), a selective inhibitor of cAMP-dependent protein kinase, reduced the FSK-mediated response. The level of CgB was also increased in FSK-treated cells, as evidenced by immunofluorescent analysis which showed a more intense staining in PC12 cells treated with FSK for 48 h than in untreated cells. The intensity of SgII staining was diminished by FSK treatment, most likely a result of a decreased rate of synthesis as well as an increase in the release of SgII. This study demonstrated that the mRNA and protein levels of CgB and SgII are differentially regulated by cAMP in PC12 cells.

Parathyroid hormone-related protein, chromogranin A, and calcitonin gene products in the neuroendocrine skin carcinoma cell lines MKL1 and MKL2

We studied the production of parathyroid-hormone-related protein, chromogranin A, calcitonin and calcitonin-gene-related peptide in the neuroendocrine skin cell line, MKL1, and a subsequently derived cell line designated MKL2. Both cell lines had cytological, histological and electron-microscopic features typical of neuroendocrine differentiation. Immunohistology and radioimmunoassay studies demonstrated the presence of parathyroid-hormone-related protein, chromogranin A, calcitonin-gene-related peptide, and calcitonin in the MKL2 cell line and the last three substances in both cell lines. The secretion of each of the first three substances was regulated by phorbol in the MKL2 cells. Additional immunohistochemical studies demonstrated the variable expression of bombesin, substance P, and vasoactive intestinal polypeptide in MKL2 cells, and the expression of synaptophysin in both MKL1 and MKL2 cells. These studies demonstrate the neuroendocrine characteristics of the MKL cell lines and provide a novel model for studies of the production and interactions of several neuroendocrine proteins and peptides by human skin cells.

Distinct patterns of chromogranin A-related species can be demonstrated in endocrine cells

We have studied the pattern of chromogranin A (CgA)-related species in different human endocrine cells that produce CgA and also express the calcitonin gene. Antibodies against CgA peptides that span its linear sequence were used in Western analysis of cell lines derived from medullary thyroid carcinoma (MTC), small cell lung cancers (SCLC), epidermoid cell lung cancer (ECLC) and a pulmonary carcinoid tumor (CRND). Each of the cell lines demonstrated a distinct pattern of CgA-related species. Gel filtration studies also revealed multiple and different forms of immunoreactive CgA in the cell lines. Although proteolysis may contribute to our results, these observations suggest that native CgA is processed to smaller species in a tissue-specific pattern by different endocrine cells. More conclusive studies, however, are necessary to establish that cell processing leads to the specific CgA moieties that we have observed.